This invention relates to battery operated inverters for converting low voltage direct current into high voltage alternating current and, in particular, to an inverter for driving a plurality of EL lamps separately.
As used herein, an EL “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.” An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include phosphor particles or there can be a separate layer of phosphor particles adjacent the dielectric layer. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
A suitable voltage can be obtained from an inverter using a transformer. For a small panel, a transformer is relatively expensive. The prior art discloses several types of inverters in which the energy stored in an inductor is supplied to an EL lamp as a small current at high voltage as the inductor is discharged, either through the lamp or into a storage capacitor. The voltage on the lamp or the capacitor is pumped up by a series of pulses from the inductor. Capacitive pump circuits are also known but are not widely used commercially.
The high voltage direct current must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,527,096 (Kindlmann) discloses a switching bridge for this purpose. The bridge acts as a double pole double throw switch to alternate current through the EL lamp at low frequency. U.S. Pat. No. 5,313,141 (Kimball) discloses an inverter that produces AC voltage directly. A plurality of inverters are commercially available using either technology.
It is known in the art to power more than one EL lamp from a single driver; e.g. see the data sheets for Supertex HV831 or HV858 lamp drivers. In these drivers, one half of a full bridge is coupled to each output. The EL lamps have one terminal in common, which is connected to a common half bridge. A serial interface is disclosed for selecting outputs. The brightness of the lamps is externally controllable in the HV858 lamp driver using a three bit code (seven levels of brightness and off).
In the patent literature, U.S. Pat. No. 6,121,943 (Nishioka et al.) discloses a plurality of half bridge outputs coupled to one electrode of EL lamps and a single half bridge coupled in common to the second electrodes of the EL lamps. U.S. Pat. No. 6,867,755 (Ashizawa et al.) discloses a single integrated circuit coupling alternating current to a plurality of EL lamps. U.S. Pat. No. 7,109,954 (Wong et al.) discloses drivers that can be cascaded and separately addressed for driving a display having plural segments, wherein each segment is an EL lamp. These patents relate to displays in which the EL lamps are pixels and have an area that is a small fraction of a square centimeter. The lamps are not used for backlighting. This invention relates to lamps used for backlighting.
As known in the art, the current drawn by an EL lamp, while small, is not insignificant and is approximately proportional to area. Thus, the size of the lamps affects loading, which affects brightness. Lamps used for backlighting must be driven in a way that does not cause one lamp to be noticeably brighter or dimmer than another. While it is known in the art to provide a few levels of dimming, it is desired to have a larger number of levels, e.g. corresponding to five bits of data (thirty-one levels and off), for better matching.
In view of the foregoing, it is therefore an object of the invention to provide an EL driver capable of selectively powering a plurality of EL lamps, wherein each lamp has an area greater than approximately one square centimeter.
Another object of the invention is to provide an EL driver compatible with standard serial interface protocols for controlling dimming and enable.
A further object of the invention is to provide an EL driver that can be configured to at least three addresses.
Another object of the invention is to provide an EL driver having plural outputs wherein each output includes five bit level control.